1. Technical Field
This invention relates generally to laser digital thermal dye transfer printers, and more specifically to such printers using combined pulse-width and amplitude modulation of exposure source power.
2. Background Art
Any type of light source such as incandescent bulbs, arc lamps, flash lamps, and light-emitting diodes might be used to supply the writing beam for thermal dye transfer. Each writing beam in laser thermal dye transfer must deposit at least a threshold amount of exposure, denoted by H.sub.threshold, at a location (x,y) on a donor in order to remove a noticeable amount of image dye from that location, possibly transferring a portion of the removed dye to an adjacent receiver. Since H.sub.threshold typically decreases with increasing beam irradiance, decreasing beam width, and increasing scanning velocity, laser beams are the most commonly used light sources.
The specific value of H.sub.threshold may be dependent upon the chemical identities of the donor's constituents, their amounts, and their relative placement within the donor; and upon the strength, spectral content, spatial distribution, and temporal duration of the exposing beam. In the case of a scanned exposure, the local scanning speed and the local scan line spacing of each exposing beam also affects H.sub.threshold.
The amount of dye transferred from a location (x,y) on the donor is approximately linearly proportional to the exposure effected at that location on one pass of a scanning writing beam if the laser thermal dye transfer is performed "near the adiabatic limit". Laser thermal dye transfer is said to be performed "near the adiabatic limit" if time T.sub.traverse required by the scanning beam to traverse a distance on the donor equal to the beam's width at that location on the donor is significantly less than the time required for the heat generated by that beam's absorption in the donor to diffuse a distance equal to the smallest dimension of the donor volume being heated at one instant by the beam. The exposure H deposited by the beam during the time T.sub.traverse is the beam's irradiance accumulated during this traversal time T.sub.traverse. The beam's irradiance at a location (x,y) is the product of the beam's power Pt!, which may be modulated or extinguished during the traversal time, and the spatial distribution pattern .GAMMA.x.sub.beam,y.sub.beam ! of that light power in the reference frame of the scanned beam at location (x,y) in the donor so that the exposure can be expressed as: ##EQU1##
In digital laser thermal dye transfer printers, the average density of an image pixel may be varied by changing the power of the exposure source (referred to as "amplitude modulation"). Alternatively, the average density of an image pixel produced by digital laser thermal dye transfer printers may be varied by changing the time during which the exposure source is ON (referred to as "pulse width modulation") while maintaining the source at a fixed optimum power, such as disclosed in commonly assigned U.S. Pat. No. 5,241,328, which issued to S. Sarraf et al. on Aug. 31, 1993. As used herein, phrases such as "exposure source" "exposure beam source" or the like are intended to refer to a radiation source 7 that can be modulated internally of the the radiation generator, such as for example by controlling the electrical power to a laser as shown in FIG. 1A, or to a radiation source 7' that can be modulated externally of a radiation generator 17 having a constant power supply 21, such as for example by controlling the input to an acousto-optic modulator 19 through which a laser beam is transmitted as shown in FIG. 1B.
While the pulse width modulation system disclosed in the Sarraf et al. patent provides a linear change in image density (tone scale) with changes in arbitrary exposure, this is not always the most desired result. In some applications, a non-linear relationship between exposure and density is desired for detailed representation of certain aspects of image content.